questions without doing too many complicated calculations. Water is roughly 1,000 times denser than air, [*] so if you want to know whether something could float if you filled it with helium, just estimate how heavy it would be when filled with water, then move the decimal point over 3 places. That’s how much buoyancy it could produce, so it’s how light the solid parts have to be in order to float. For example, a fish tank full of water might weigh 150 kilograms. That means that it displaces about 0.150 kilograms of air, or 150 grams, which is about the weight of a large smartphone. Since an
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